European patent EP 0 487 920 describes a device of the above-mentioned type in the form of a PTC element which at least comprises one polymer-based electrically conducting body, the resistivity of which exhibits a positive temperature coefficient and electrodes for conducting current through the body. At least one of the two contact surfaces of the polymer body makes free contact with one of the electrodes or another electrically conducting body. The polymer body and the electrodes are retained by resilient and pressure-applying devices. This results in to the creation of a contact pressure in a contact surface where the polymer body freely makes contact with an electrode or another electrically conducting body. Since current transition between the electrode and the polymer body only takes place at certain contact points, a current displacement arises at the contact surface resulting in a voltage drop near the contact surface. As a result of the difference in resistivity between the polymer body and the electrode, this voltage drop occurs substantially in the body which has the highest resistivity, that is, the polymer body.
At current intensities below rated current, a low contact resistance is maintained at the contact surface in that the contact pressure which is applied to the contact surface by the pressure-applying devices ensures current transition at a sufficient number of contact points. At short-circuit currents, the temperature at the contact points is increased. This increase in temperature results in a local melting and/or gasification of the polymer material at some of these points, whereby the resistance is increased. Finally, a layer of gasified polymer/carbon arises near a freely contacting contact surface, whereby a strong increase of the resistance across the layer is obtained, that is, the current-limiting device trips. Since the contact pressure is maintained by the pressure-applying devices, the original contact pressure and the original contact resistance are essentially resumed across the contact surface in connection with the gas pressure decrease. One problem is that the gasification has normally arisen at both contact surfaces of the polymer body before the current-limiting device trips. Therefore, it is difficult, using constructive changes such as cooling electrodes, etc., to increase the ability of the current-limiting device to withstand overcurrents for a limited period of time, i.e, currents with a current intensity is normally up to 10 times the rated current, while at the same time maintaining the ability of rapidly limiting short-circuit currents. Overcurrents of the order of magnitude of 10 times the nominal current occur frequently, for example when starting electric machines.
To maintain a low contact resistance at a normal current transition and to ensure that essentially the original contact and the original contact resistance are formed again after the current-limiting device has tripped as a result of a short-circuit current, it is desirable that a great contact pressure act on the contact surface since a high contact pressure ensures a large number of contact points and hence a large effective contact surface and a small current displacement. However, at the same time, the current-limiting function of the device is deteriorated since the limited current displacement results in a low energy development in the layer. In this way, with increasing contact pressure, a larger level of power must be supplied to the polymer body before the current-limiting device trips. This means that an increased contact pressure extends the time to a trip at short-circuit currents, which cannot be tolerated.
In European patent EP-A3-454 422, a PTC element is described which comprises a polymer-based electrically conducting body, the two contact surfaces of which consist of a polymer composition comprising an electrically conducting metal powder, preferably a nickel powder. By the addition of nickel powder, a PTC element with integrated electrodes is obtained. For a PTC element according to EP-A3-454 422 with a low contact resistance at both of its contact surfaces and hence a small current displacement at the contact surfaces, the whole of the polymerbased element will be heated homogeneously at increased current transition. This extends the time to trip at short-circuit currents in the same way as described above for a PTC element with a high contact pressure at the contact surfaces. Since the energy development in the PTC element becomes evenly distributed over the whole bulk of the PTC element, a large energy volume must be supplied to the PTC element before it trips. This means that the local melting and/or gasification of the polymer material, which arises at short-circuit currents and greatly increases the resistance and leads to the current-limiting device tripping, is not concentrated at a layer but arises at points, so-called hot spots, distributed over the whole bulk of the PTC element. In this way, there is no possibility, with a PTC element according to EP-A3-454 422, to predict where in the electrically conducting body the gasification occurs. The low contact resistance and the low power loss at a normal current transition for a PTC element according to EP-A3-454 422 are thus obtained at the cost of a long time to trip at short-circuit currents when a large energy volume must be supplied to the PTC element before it trips.
The object of the invention is to provide a device for current limitation, comprising an electrically conducting body which exhibits a low resistance at current intensities below rated current and an increased resistance to the overcurrents which normally occur in electric installations, that is currents with a current intensity which normally is up to 10 times the rated current, while at the same time the ability to rapidly and reliably limit short-circuit currents and other large fault currents is maintained or improved.
Further, it is an object of a current-limiting device according to the invention to be able to predict that tripping occurs at a layer near one of the two contact surfaces of the electrically conducting body and in which of these two contact surfaces tripping occurs. In this way, constructive measures can be suggested, by means of which the resistance across the electrically conducting body can be reduced, the resistance to brief and limited overcurrents increased, and the ability to rapidly and reliably limit short-circuit currents improved.